On the D54 that I own, I don't have to move the action prior to cocking. The cocking lever will reset the action to where it is needed. As Artie says, the cocking lever will move the action to a point that seems to be fully cocked, but it has a little bit more to go. You do not have to hold the action to cock the gun. Be sure that you pull the lever to a fully cocked position.It sounds like you have a 'droop' barrel. You must decide if you want to keep the gun you have or return it for the 'Droop' barrel defect. If you want to keep it, you can bend the barrel or modify the sights. I bent the barrel on my drooper D54. There are threads here about bending barrels, do a search. On my 54, I removed the D54 action from the stock, and used 2x4 pieces of lumber to make a kind of jig to hold the action and bent the barrel by putting my weight on the barrel. it worked for me, but YMMV. I have a Diamond Back scope by Vortex on my 54, no problems.
Hector Medina is the subject matter expert when it comes to the D54. You may want to consult him prior to a scope purchase. At one time he recommended the Vortex Diamondback 4-12x40 A0. That scope is now out of production but NOS D-backs occasionally pop up on EBay.My new 54 shipped with a Hawke scope which the gun promptly ate in 14 rounds. Unlike the Hawke scope, Vortex scopes come with a lifetime transferable warranty. I currently have a second generation 30mm ZR mount with a BSA non-air gun rated side focus scope on the 54. It's holding up fine. But I do have a Diamondback with UTG compensation mount ready for backup. Like the Diamondback, the ZR mounts are hard to come by but more so. The ZR is droop compensated as well.Food for thought...